This invention relates to the production of aluminum chloride. More particularly, this invention relates to an improved process for the control of particle size and purity of aluminum chloride.
In the production of aluminum chloride suitable for subsequent electrolytic reduction to metallic aluminum by the chlorination of materials containing compounds of aluminum as well as other materials such as silicon, titanium, and iron, the resulting chlorides must be separated to provide a sufficiently high purity aluminum chloride for the subsequent electrolytic process to perform in a satisfactory manner. In King et al. U.S. Pat. No. 3,786,135 there is disclosed and claimed a process for the recovery of high purity aluminum chloride from the gaseous effluent of chlorination of aluminum compounds which involves a first step of initially cooling the hot gaseous effluent sufficiently to selectively condense sodium aluminum chloride and other high melting point chloride values therefrom and separating such initially condensed values as well as entrained particles from the gaseous effluent followed by a further cooling of the gaseous effluent to a second and lower predetermined temperature range to condense a high proportion of the remaining volatile constituents that are condensable above the condensation temperature of aluminum chloride. The final step claimed in that process relates to the direct desublimation of high purity aluminum chloride values in a fluidized bed of aluminum chloride at a temperature range of from about 30.degree.-100.degree. C. It is in the area of this third step that the refinements comprising the process of this invention are directed.
In the aforesaid patent there is illustrated a fluidized bed containing fluidized particles of aluminum chloride into which the vapors are passed at an undisclosed velocity. The vapors are said to pass through the fluidized bed at a temperature of about 30.degree.-100.degree. C to provide condensation of the vapors on the solid aluminum chloride particles. Filters above the fluidized bed prevent the loss of particles, particularly very fine particles, from the condenser. Provision is illustrated for removal of the solid aluminum chloride from a position adjacent the bottom of the condenser. As mentioned above, the operating temperature within the condenser is stated to be from 30.degree.-100.degree. C, suitably within about 60.degree.-90.degree. C and preferably within the narrower range of 50.degree.-70.degree. C. The patentees go on to describe the effect on particle size of the condensation temperature noting that at lower temperatures within the specified range of 30.degree.-100.degree. C the average particle size of the condensed product is generally smaller. The patentees further note that even within the range of 30.degree.-100.degree. C, a certain amount of the gaseous aluminum chloride values will not desublime. They, therefore, indicate the desirability of using condensation temperatures at the lower end of the stated range of 30.degree.-100.degree. C.
While operation of the condensation process at the lower end of the range as taught in the King et al. patent does result in a satisfactory particle size as well as an economically attractive yield of aluminum chloride, it has been found that such operation can lead to undesirable condensation of the by-products such as titanium tetrachloride. Furthermore, since the filing of the aforementioned King et al. patent in 1971, more has been learned as to the operating conditions within the fluidized bed during condensation.
While it would appear that simply raising the temperature of the condensation would eliminate the contamination problem, it has been discovered that other operating parameters, particularly entrance velocity, must also be controlled.
Thus, while two of us have described and claimed a process for controlling impurities using a first high temperature fluidized bed followed by a second lower temperature fluidized bed in patent application Ser. No. 765,459, entitled "Method of Producing High Purity Aluminum Chloride", filed Feb. 3, 1977, we have discovered that not only control of temperature is important but control of the entrance velocity and the method of removal of the condensed aluminum chloride must be considered whether operating with one or two fluidized beds.